The present invention relates to a device for measuring the rate of flow of a preferably multi-phase stream. The device conventionally has a shaft-mounted impeller wheel which is driven with a constant rpm and on which the flowing mass axially impinges. The impeller wheel deflects the flowing mass and imparts thereto a radial and an axial velocity component. The shaft of the impeller wheel carries a driving spur gear which meshes with an intermediate spur gear held stationarily by a force measuring device. The intermediate spur gear also meshes--diametrically opposite from the driving spur gear--with a driven spur gear rotated by a drive motor.
Such devices are employed for measuring the throughput of bulk material, particularly powdered or fine-grained substances. The bulk material is guided axially onto the impeller wheel having radially oriented guide vanes, and the driving torque of the synchronous motor is measured. A change in the torque caused by the deflection of the axially introduced bulk material is a direct measure for fluctuations in the stream of bulk material.
In all such devices, the main problem is to measure the driving or braking torque with sufficient accuracy and with particular consideration of the fact that the shaft of the rotating element is decelerated not only by the stream of bulk material but also by friction forces generated in the bearing of the drive shaft itself and also in the drive assembly of the measuring device. These friction forces change considerably upon fluctuations in temperature since the viscosity of the lubricants required for the shaft bearing changes.
German Pat. No. 3,507,993 discloses a compensation of interfering torques originating in the measuring device by mounting the intermediate shaft of a spur gear that drives the rotating element and is disposed in a carrier element so that it is pivotal about the drive shaft of the gear assembly which is coaxial with the rotating element. The pivotal movement of the intermediate shaft and the carrier element are limited by a force measuring device.
The prior art measuring device outlined above has been found satisfactory down to a measuring range for a bulk stream of 50 kg/min; for smaller mass streams the sensitivity of the measuring device was no longer sufficient.